An insulated gate bipolar transistor (IGBT) is a semiconductor power device with a compositing structure that combines features of a metal-oxide-semiconductor field effect transistor (MOSFET) and a bipolar junction transistor (BJT). Performance-enhancing features of an IGBT are designed to achieve a higher current density than a MOSFET, and faster and more efficient switching characteristics and better control than a BJT. Additionally, the drift region of the IGBT can be lightly doped for improved blocking ability. Meanwhile, an IGBT device can still have good conductivity because the lightly doped drift region undergoes high level carrier injection from a bottom P collector region resulting in conductivity modulation. With the MOSFET's characteristic of easy control with a gate electrode, the bipolar current flow mechanism and the advantages of shorter switching time and lower power loss, the IGBT is widely applied in a high voltage and high power application.
Conventional technologies to configure and manufacture IGBT devices are still confronted with difficulties and limitations to further improvement in performance due to various tradeoffs. In IGBT devices, there is a tradeoff between conduction loss and turn-off switching losses, Eoff. Conduction loss depends upon the collector to emitter saturation voltage Vce(SAT) at rated current. Greater carrier injection while the device is on improves the conductivity of the device, thus reducing conduction loss. Increased carrier injection would, however, cause higher turn-off switching losses because of energy dissipated in clearing out injected carriers during turn-off.
Another trade-off exists between the IGBT's collector-emitter voltage at saturation (Vce(SAT)) and its breakdown voltage (VBD). While an increase on topside injection may improve Vce(SAT), it usually comes at a cost of lowering breakdown voltage VBD. An IGBT device with a high density of deep trenches may overcome this trade-off, but it is hard to make such device with a high density of small pitch high aspect ratio trenches.
Various configurations of IGBT devices have been developed in recent years. FIG. 1A is a cross sectional view of a conventional IGBT device 100A. In the example shown in FIG. 1A, a heavily doped N layer 102A is disposed below the channel region 103A and at the top of the lightly doped drift region 101A to further enhance the carrier injection on the topside. However, such a device has a lower breakdown voltage and has a high Crss capacitance due to the heavily doped N layer 102A. The high Crss capacitance of the IGBT device may slow down the device switching speed and lead to higher switching energy loss.
FIG. 1B is a cross sectional view of a conventional IGBT device 100B having a planar gate 136 with a trench shield electrode configuration. The IGBT device 100B is formed in semiconductor substrate 105 that has a first conductivity type, e.g., a P type substrate 105. An epitaxial layer 110 of a second conductivity type, e.g., an N-epitaxial (epi) layer 110, is supported on top of the P-type substrate, 105. A collector electrode 120 disposed on a bottom surface of the substrate. In this type of device, shield trenches 135-S have a shield electrode 137 surrounded by dielectric (e.g. oxide) 126. The shield trenches 135-S in the device 100B do not have a gate electrode component. Instead, a planar gate 136 is disposed on planar gate oxide 125-P that insulates the planar gate 136 from the semiconductor surface. A shield electrode 137 is connected to the source/emitter voltage. In this example, the channel is generally horizontal, running at the top of the body region 140, beneath the planar gate 136, from the source 130 (and optional lightly doped source 131) to the top of a heavily doped N+ region 145. This embodiment may be easier to manufacture, as it is simple to form a planar gate and because the shield trench 135-S with its single electrode is much easier to form than a shield gate trench structure with multiple electrodes. The shield trench 135-S still charge compensates the N+ region 145 to keep the breakdown voltage (BV) high, and also keeps the capacitance Crss low for fast and efficient switching. While device of the type shown in FIG. 1B may achieve a reduced Crss and increased injection with lower Eon and Eoff losses, it requires high density deep trenches. In addition, the heavily-doped N region 145 may degrade the breakdown voltage.
FIG. 1C is a cross-sectional view of another IGBT with partially narrow mesa in the 3rd dimension. With the configuration of the narrow region between the gates such configuration, the injection enhancement can be increased. However, such a device requires complicated design and process. An example of such a design and process may be found in M. Sumitomo, J. Asai, H. Sakane, K. Arakawa, Y. Higuchi, and M. Matsui, “Low loss IGBT with Partially Narrow Mesa Structure (PNM-IGBT),” Proceeding of the 2012 international Symposium on Power Semiconductor Devices and ICs, page 17, 2012.
There exists a need to develop an IGBT configuration without having high-density deep trenches or complicated design/process so as to reduce cost and improve performance without sacrificing breakdown.
It is within this context that aspects of the present disclosure arise.